Data storage systems often utilize several storage devices such as hard magnetic disk drives to provide adequate permanent data storage capacity. Each hard disk drive incorporates a multiple-disk stack mounted on a spindle and driven by a motor operated at a predetermined rotational speed; this arrangement is commonly referred to as a spindle-disk assembly or SDA. Data transfer to and from each disk drive is effected by means of a plurality of read/write transducers, such as magnetic read/write heads, each one of which is associated with a disk surface and is mounted on a common actuator mechanism. Each disk drive also typically has an associated spindle motor speed control circuit to maintain the rotational speed of the SDA within predetermined limits. 3600 RPM is a common rotational speed, but the trend is toward higher RPMs in order to increase the rate at which data can be read from or written to the disk drive.
When a number of independent disk drives are employed in a storage system, it is advantageous to synchronize their operation. In particular, it is advantageous to have precise phase alignment between disks in different drives in order to minimize unnecessary delays when switching from a disk in one drive a disk in another. This need for synchronism requires a higher degree of rotational control than is achieved by a set of independent speed controllers.
One known technique for achieving synchronous operation is to phase-lock the rotation of each disk drive to a single master reference signal. For example, it is possible to phase-lock a feedback signal indicative of spindle position to the master reference signal, and thereby phase-lock the spindle to the master reference signal. Such a scheme without further modification has only coarse phase-alignment resolution, because generally the feedback signal only indicates spindle position to the nearest rotor pole. Also, it still requires some way of aligning the information on the disk with the spindle, so that phase-aligning the spindles results in phase-aligned disks. This requirement burdens the manufacturing step of "servowriting" i.e., writing servo-mechanism-related information to the disk, in at least two ways: 1) the servowriter must somehow track the spindle motor rotation to ensure that the position information is aligned with the spindle; and 2) the servowriter and final product are constrained to use similar if not identical spindle controller hardware, which may be disadvantageous under some circumstances.
Another technique for achieving synchronous operation is to phase-lock the master reference signal with a disk-position signal that is derived from a pattern recorded on a disk in the disk drive. Because this technique relies on a disk-position signal rather than a spindle-position signal, it overcomes the above-mentioned problems of using spindle alignment. However, it also requires the continuous presence of the disk-position signal. In so-called "dedicated servo" systems, which have a dedicated disk surface for providing disk-position information, this requirement is not a problem. However, in modern "embedded-servo" systems, which obtain position information from data surfaces rather than a dedicated surface, the disk-position signal is occasionally interrupted, for example during head switches. After an interruption, the drive must re-acquire phase lock before disk accessing can continue, resulting in undesirable delay.
One example of synchronizing multiple disk spindles is shown in U.S. patent application Ser. No. 07/721,044 filed Jun. 26, 1991, entitled "Disk Storage System", inventor Even, assigned to Digital Equipment Corp. That system employs a modified version of the disk-position feedback system described above. It includes a trigger pulse modifier as part of the phase lock servomechanism. During startup operation of the disk drive, the trigger pulse modifier measures the phase difference between the master reference signal and a sector zero reference pulse generated from a reference pattern recorded on the disk. This phase difference is applied to a lookup table, the output of which is an integer indicating a number of cycles of phase shift to be applied to a sensor signal generated by sensors on the spindle. The phase-shifted signal is then applied, after suitable frequency division, as a feedback signal to a phase difference detector in the loop.
This system has the advantage of taking disk position into account during phase locking. And since the pulse modification only takes place during the startup phase, it does not rely on continuous disk position information. However, like the spindle-lock scheme it requires that the reference pattern must be aligned with the spindle, and also that the servowriter and final product use a similar controller. An improved technique, therefore, would achieve similar beneficial results while eliminating or substantially reducing these constraints, thereby simplifying the servowriting process and providing greater design flexibility.